1. Field of the Disclosure
The present disclosure relates generally to a method and apparatus for extracting interference signal information without additional signaling from a network in a communication system.
2. Description of Related Art
Generally, in a cellular-based communication system, a terminal (e.g., a User Equipment (UE), a Mobile Station (MS), etc.) may suffer from interference caused by a signal from another terminal that uses the same resources in the same cell, and/or from another terminal in an adjacent cell. In this case, the terminal may detect or remove the interference signal from the signal that the terminal should receive, using an interference detection technique such as joint detection, thereby improving its signal reception performance.
However, if the terminal demodulates and decodes only the target signal without information about the interference signal, performance degradation caused by the interference signal may increase, and this phenomenon may be more severe at the terminal in the boundary of the cell. To combat this, the terminal may obtain control channel information for the interference signal, and use it for demodulation and decoding. In this case, however, the terminal should separately receive information about the interference signal from a base station (e.g., an evolved Node B (eNB), etc.) or should perform blind detection. Using blind detection, the terminal may reduce false alarms or misdetection probability with filtering techniques, but it is difficult to make correct filtering determinations, thereby limiting performance improvement of the interference cancellation function.
Generally, when terminals attach to or connect to a base station, the base station allocates a unique ID to each of the terminals (hereinafter, referred to as a UE-ID). However, each terminal may not know a UE-ID of another terminal since the terminal receives only its own UE-ID through upper-level signaling. In the 3GPP LTE system, the UE-ID is called an RNTI.
FIG. 1 illustrates an application of a UE-ID to a control channel in a conventional LTE system.
Referring to FIG. 1, a base station generates DCI, and then attaches a 16-bit Cyclic Redundancy Check (CRC) to the DCI, for error detection at a terminal. That is, to distinguish a DCI of each terminal, the base station may mask a 16-bit CRC with a UE-ID through an Exclusive OR (XOR) operation, and then transmit the masking results over a control channel (e.g., a Physical Downlink Control Channel (PDCCH)).
A terminal may receive a control channel in every subframe, and then find its own DCI through a blind decoding process in which a terminal attempts decoding for all wireless resource units that are available for each terminal in a control channel. For the decoded control channel signals, the terminal determines whether a DCI in the decoded control channel signals is its own DCI, using its unique UE-ID.
FIG. 2 illustrates a conventional method of a terminal receiving a control channel signal.
Referring to FIG. 2, the terminal decodes a received DCI, and then generates a CRC with information bits, excluding 16 tail bits corresponding to a CRC. If the decoded DCI matches a DCI of the terminal, a UE-ID of the terminal may be derived when the XOR operation is performed on the decoded tail bits and the CRC generated by the terminal.
Therefore, if the UE-ID derived through the XOR operation matches the UE-ID of the terminal (i.e., Success), the terminal may demodulate and decode received data using the DCI information, determining that the decoded DCI is its own DCI. However, if the UE-ID derived through the XOR operation is different from the UE-ID of the terminal (i.e., Fail), the terminal may discard the DCI, determining that the decoded DCI is a DCI of another terminal.
FIG. 3 is a flowchart illustrating a conventional control channel decoding process in a terminal.
Referring to FIG. 3, a terminal receives and demodulates a control channel in step 301. In step 303, the terminal decodes the demodulated control channel. In step 305, the terminal calculates a CRC using information bits of the decoded control channel. In step 307, the terminal determines whether a value determined, i.e., a determined UE-ID, by performing the XOR operation on the decoded tail bits and the calculated CRC matches its own UE-ID previously received from the base station. If the determined value is the same as the terminal's own UE-ID, the terminal demodulates and decodes information of a data channel using the DCI, determining that the decoded data is its own DCI, in step 311. However, if it is determined in step 307 that the determined value is not the same as its own UE-ID, the terminal discards data of the decoded control channel, determining that the decoded data is a DCI for another terminal, in step 309.
After obtaining DCI information, the terminal may receive downlink data from a base station in a cell to which the terminal belongs. However, if there is a terminal that uses the same frequency-time resources in the same cell as that of the terminal, or if a base station of another cell is transmitting data to another terminal using the same frequency-time resources, the terminal may experience performance degradation due to the interference problems, when receiving data. Although various methods have been proposed to solve the interference problems in a terminal, the actual performance improvement is limited if the terminal does not have information as to whether a signal is an interference signal.
For example, if a terminal has correct information about an interference signal, the terminal may improve the reception performance using a method of joint-detecting the signal the terminal should receive, and the interference signal. However, because a terminal generally does not know information about UE-IDs of other terminals, the terminal may not extract a DCI of another terminal, in which information about the interference signal is present, so the terminal may not use an improved algorithm such as joint detection.
Basically, in order for a terminal to know a UE-ID in an interference signal, the base station should provide information about the UE-ID through separate signaling, or the terminal should directly detect the UE-ID. However, if the base station does provide this information about the UE-ID in the interference signal, this will increase the overhead of the control channel. Accordingly, to address this issue, a blind decoding scheme has been proposed, in which a terminal attempts decoding for allocation of all possible control channels, and determines the validity of the control channel using a soft metric and the like.
In the existing LTE system, a blind decoding method for extracting a terminal's own control signal has limited the complexity by allowing the terminal to attempt to decode only 44 detection locations, by limiting the detection locations using its own RNTI that the terminal already knows. However, if the blind decoding scheme used for decoding a control channel of another terminal is applied, the terminal should perform decoding for all the detection locations of the full band and the DCI formats because the terminal does not know the RNTI of the other terminals, increasing the likelihood of RNTI false alarms.
Specifically, in the LTE system, DCI information of each terminal may be transmitted over a PDCCH including a plurality of Control Channel Elements (CCEs), and the PDCCH may be divided into four types of Aggregation Levels (ALs) and into a plurality of DCI formats depending on the number of CCEs allocated to the terminal. Therefore, if there are a total of, for example, 43 CCEs, there are a total of 79 PDCCH candidates (including 43 PDCCH candidates for AL=1, 21 PDCCH candidates for AL=2, 10 PDCCH candidates for AL=4, and 5 PDCCH candidates for AL=8). If 6 formats exist for each of the number of DCI cases, 474 candidates may be present in the DCI information that is finally allocated to one terminal.
Therefore, a terminal may blind-decode all possible PDCCH candidates to obtain a UE-ID and control channel information of another terminal, and may use a soft metric-based filtering or UE-ID based filtering method to decrease the false alarm probability of falsely estimating a UE-ID.
The soft metric-based filtering method may use reliability information of decoded data. If decoding is performed on all possible DCIs, information about decoded data may be provided from a decoder. The decoded data may be re-encoded to define a difference or correlation between the re-encoded data and input data as a reliability value, and the reliability values for all possible DCIs may be calculated in order to determine whether a PDCCH is valid for the DCIs having a high reliability value. That is, in a good wireless channel environment, if a DCI has valid information, a DCI value having a very high reliability value may be calculated through the decoding and re-encoding process. However, in the soft metric-based filtering scheme, even though a soft metric value is large, a false alarm other than a desired UE-ID value may be generated. In particular, if an AL is low, a false alarm is likely to occur.
The UE-ID based filtering method uses CCEs of a PDCCH determined by a UE-ID of a terminal. In the LTE system, for a PDCCH having information about each terminal, locations of CCEs may be determined by the UE-ID (i.e., RNTI) value of the terminal. If there are 43 available CCEs and AL is 1, 2, 4, or 8, a PDCCH may start at one of 6, 6, 2, or 2 CCE locations depending on the AL value, respectively. As a result, it is possible to determine whether the DCI and RNTI are valid information. For example, assuming that a CRC is calculated using the results obtained by decoding a PDCCH that has AL=1 and is located in CCE index=5, and a UE-ID value determined by performing the XOR operation on the CRC and the tail bits is represented by X, if CCE indexes which are possible with X are {7, 8, 9, 10, 11, 12}, the CCE index=5 of the decoded PDCCH may not be included in a set of CCE indexes possible with X. Thus, a UE-ID X=5 would be considered invalid, and the UE-ID and DCI information may be discarded. However, in the UE-ID based filtering scheme, multiple RNTI candidates still exist, even after undergoing filtering, so a possibility of the false alarm is high. For example, assuming that there are four types of ALs (AL=1, AL=2, AL=4, and AL=8), there are six types of DCI formats, and there are 43 available CCEs, then the total number of possible PDCCH candidates is 474. If the UE-ID based filtering is applied thereto, 96 PDCCH candidates may remain on average. If the soft metric-based filtering is additionally performed, 16 PDCCH candidates may remain on average. That is, even though both of the current two techniques are used, the false alarm possibility of falsely estimating a UE-ID is high.